1. Field of the Invention
The embodiments of the present invention relate to sensors for measuring magnetic field and electric field phenomena at the same time at the same point in space.
2. Background of the Art
Conventional small magnetic field sensors (commonly referred to as B-dots) consist of a coaxial cable with a coil located at the cable end. The center wire of the coaxial cable extends beyond the outer shield and is shaped in the form of a coil. Typically, the coil may be a single half loop, an integer number of loop turns, or an integer number of half loop turns. The coil end is then typically soldered to the outer shield. A conventional differential B-dot makes use of two nearly identical coils spaced closely together in a nearly unique orientation. For packaging purposes, the differential B-dot is housed within a conductive block filled with a dielectric substance. This packaging is not necessary in the differential B-dot design except when trying to eliminate or control proximity effects. Interconnection is made with two individual pieces of transmission line (coaxial cables). The B-dot is used to measure the rate of change of the magnetic fields in the location of the loop. According to Faraday's law, as the magnetic field lines threaded through the B-dot loop changes with respect to time, a voltage is induced in the coil that in turn drives a coil current. In an approximate sense, the induced voltage is proportional to the number of loops, the area of the loop, the magnetic field strength and either the inverse of time duration of the change or the time harmonic frequency of the signal.
More specifically, the voltage induced in a coil is proportional to the frequency of the event, the total-cross section of the coil and the number of turns. Thus, to obtain reliable sensitivity at lower frequencies, it is necessary to increase the cross-section and/or the number of turns. On the other hand, to widen the response to high frequencies, it is necessary for the coil to have low stray capacitances (capacitance between the coil and external entities) and low internal capacitances (capacitance between coil turns). Consequently, a reduced number of turns is desired and the dimensions of the sensor should be small compared with the wavelength corresponding to the highest frequency of interest. Therefore, it is difficult to fabricate a sensor having a wide pass band and with reliable sensitivity throughout the pass band.
A shielded flat coil is also known in the art. The shield is not continuous thereby avoiding a short-circuit loop which would generate a current in response to changes in the external magnetic field. This current would just counter the effects of the external field that the internal sensor would not detect the external field. A flat coil sensor can be optimized either for reliable sensitivity at low frequencies with a large diameter and a large number of turns, or for a response at high frequencies with a small diameter and a small number of turns. Unfortunately, the first optimization leads to a poor response at high frequencies while the latter optimization leads to a limitation of the sensitivity at low frequencies. Capacitive coupling from shield to shield across the gap exposing the sensor offers limitations to the sensitivity of this device.
The differential B-dot probes currently used to measure B field and exclude the E field are three-dimensional loops. This probe is a matched series parallel combination of loops which match a 50 ohm input of a tuned receiver or a power meter. This type probe must be constructed utilizing double sided flexible printed circuit board with low loss dielectric. This material is necessary to implement the complex stripline matching networks. In addition, the assembly of these loops are extremely difficult, time consuming, and the probes are difficult to maintain. The required power meters and receivers utilized to measure the output of these type loop probes are very expensive and hard to use for remote field measurements.
Two dimensional double gap detected probes are used to measure B fields in large test volumes which are remote from electrical power. Due to the large number of measurement points required to map test volumes and remoteness of some areas of interest, the probes need to be portable. In addition, the probe must be small and non-perturbing to the field being measured. The probe described in this disclosure is small, portable, and designed for minimum field perturbation. The output of the probe is read by an ordinary high impedance volt meter, which is inexpensive, small, easily portable, and does not require external power. These B-dot sensors may operate by oscillating a B-field in the loop area to induce a voltage in the conducting loop given by the relationship V=A dB/dt, where V is the voltage output of the loop, A is the area of the loop and dB/dt is the derivative of the time varying B-field. This voltage may be DC shifted by the high frequency voltage doublers and filtered by low pass filters, as described in U.S. Pat. No. 4,647,849. The DC outputs of the low pass filters are summed together by semiconductor line and brought out of the field by semiconductor lines to a voltmeter having a capacitor across it. The semiconductor lines are used to minimize the field pick-up in the transmission lines as well as minimize field perturbations.
U.S. Pat. No. 4,626,791 (“the '791 patent”) discusses B-dots and their applications. More specifically, the '791 patent recites that a B-dot sensor may act as a microwave detector. In such an embodiment, the sensor comprises a conducting metal loop placed in a microwave signal environment such that magnetic flux passing through the loop changes over time and induces an electrical signal which is then recorded. The '791 patent specifically mentions the inherent limitations of B-dot loops (column 1-2, lines 50-68, 1-9; column 4, lines 46-57). That is, they are designed to only respond to the magnetic flux. However, as set forth above, the magnetic flux is often accompanied by a voltage distribution within close proximity of the loop. The spurious signals interfere with the measurement process. The '791 patent suggests the use of a second B-dot loop oriented in the opposite direction from the first B-dot loop to eliminate the impact of the noise. According to the present inventors, the second B-dot loop will have the same amplitude and 180° phase difference. Therefore, the electric field terms reduce to zero and the magnetic signal is increased by a factor of two. In principle this is reasonable but in practice, for exact cancellation one requires (this assumes that the coil end is terminated on the grounding shield): 1. Identical probes, 2. Identical relative ground line geometry that includes line lengths, bends and twists in the line, line cross-sectional dimensions, line orientations, 3. Line and coil locations must be in close proximity (typically 1/40 of the smallest wavelength associated to the highest frequency in the band pass, 4. Coils must have exact relative 180° orientation, 5. Coils and lines must be immersed in identical mediums and have identical proximity to external structures and 6. Coil axis must be aligned. Deviations from these exactness result in spurious noise signals that may effect the overall measurement especially as the frequency is increase.
U.S. Pat. No. 4,305,705 describes a sensor to provide information about flux changes in a coil that encloses a region of changing magnetic flux is formed by placing a pair of bifilar windings in the plane of the coil for which flux change is to be sensed. The winding may be inside or outside the coil. The bifilar winding is placed along that coil, one end of the bifilar winding is terminated in a short circuit and each winding is brought out to voltage-measuring equipment at the other end. The bifilar winding limits the response to the flux produced by the coil near which it is disposed and discriminates against changes in magnetic flux enclosed within the inner diameter of the coil. Pairs of bifilar windings may be used to compare differences of voltages, and the windings may be limited to part of the circumference of the coil to make local readiness. This patent describes a B-dot coil in FIGS. 2 and 3 as a top view and a sectional side view of an EF (equilibrium field) coil 18 of FIG. 1. FIG. 3 is a sectional view along section lines 3—3 of FIG. 2. FIGS. 2 and 3 of this patent also show a sensor 20 that is placed next to and inside EF coil 18 and a second sensor 22 placed next to and outside EF coil 18. Coils such as sensors 20 and 22 are frequently referred to as “B-dot” coils to indicate that they respond to the time derivative of the magnetic flux density B.